Holey optical device

ABSTRACT

A method of making an optical device including forming a plurality of holes with varying radii milled vertically into a film, wherein said holes form a pattern. The radius of each hole determines an effective refractive index for said hole. The effective refractive index modifies a phase and an intensity of an incoming electromagnetic radiation as the radiation propagates through said hole. The device is configured to be operating equally for each linearly polarized radiation simultaneously, wherein the each linearly polarized radiation is normally incident on the device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. Patent non-provisionalapplication Ser. No. 14/431,544, filed Mar. 26, 2015 which claimspriority to, incorporates fully by reference, and is a U.S. § 371national stage entry of, International Patent Application Serial No.PCT/US2013/61917 filed Sep. 26, 2013 which is related to and claimspriority to U.S. Provisional Patent Application No. 61/707,946, filedSep. 29, 2012. All of the above applications are incorporated byreference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under N00014-10-1-0942awarded by the Office of Naval Research. The government has certainrights in the invention.

FIELD OF THE INVENTION

This invention relates to optical devices and in particular to thosedevices used in compact optical systems and other micro-scaletechnologies.

BACKGROUND OF THE INVENTION

Scaling down optical elements is essential for making compact opticalsystems. Accordingly, planar and thin meta-structures are desirable forfabrication as well as for integration simplicity. Currently,dielectric-based refraction microlenses typically used in opticalsystems have a non-uniform design, with a thickness of tens ofmicrometers. Gradient-index lenses are planar; however, theirthicknesses are about an order of magnitude larger than the wavelengthof incident light and the fabrication process is not straightforward.Classical diffraction lenses, such as Fresnel lenses, are planar andthin; however, they cannot control the phases precisely because theposition of each Fresnel ring is determined by the geometry.Phase-controlled diffractive microlenses present a combination of adiffraction lens and a refraction lens, and have been studied bothexperimentally and numerically, but their structures are not planar.

Recently, there has formed a growing interest in planar,sub-wavelength-thick metallic lenses in the optical range. Differentkinds of metallic lenses have been proposed and experimentallydemonstrated, such as the superlens, the hyperlens, surface-plasmonfocusing lenses, and superoscillation-based lenses. Some of the metalliclenses result in focus spots on surfaces, while others focus in farfield. Nanoslit lenses are one of the planar metallic lenses which aremade of arrays of subwavelength slits milled into metallic films. Eachslit width is varied to change the mode index of the single-mode lightpropagating through it, such that light transmitted through differentslits experience different phase delays. Hence, for example, by using asymmetric array of slits with decaying phase shifts relative to theoptical axis, it is possible to arrange a concave phase front, and focusa linearly polarized transmitted light. The very first designs ofnanoslit metal lenses were first modeled numerically and thendemonstrated experimentally.

The early work in this area dealt with plasmonic mode propagationthrough metallic slits. The inventors of the present invention have alsorecently shown that photonic modes can be efficiently used to introducephase delay; see, for example, Ishii, S. et al. Opt. Lett. 2011, 36,(4), 451-453, incorporated in its entirety into the present disclosure.The use of either plasmonic mode or a photonic mode enable the abilityto design polarization-selective nanoslit lenses whose focusingproperties become either a convex (light-focusing) or concave(light-diverging) lens depending on the incident linear polarizations.The focusing properties of the nanoslit lenses can be additionallycontrolled by incorporating liquid crystals inside the slits.

Although nanoslit lenses are indeed planar and quite thin, an importantdrawback of the nanoslit design is its polarization-dependence.Moreover, as a nanoslit lens is focusing light into a narrow strip, itdoes not allow for high-intensity confinement of light into awavelength-size circular spot. For some applications, the above featureshinder the reduction of nanoslit lenses to practice. Thus, there is aneed for a novel thin and planar optical device that addresses thedrawbacks identified above.

SUMMARY OF THE INVENTION

A polarization-independent optical device for bending light. The opticaldevice consists of holes with a width less than the wavelength of light,which are formed into a thin film. The holes create a specific pattern;for example, concentric rings with increasing diameter, where the holesof each ring are the same size, but the holes of each successive ringmoving outward decrease in size. The change in hole size throughout thepattern creates various phase changes, or bending angles, of the lightas it enters, proceeds through the hole, and exits on the opposite sideof the film, thus focusing or diffracting the light at a desireddistance. After the holes are milled into the film, a filler is appliedto the device in order to fill the holes and give the film a thincoating. In one embodiment the filler is a nonlinear medium allowingcontrol of the device properties via a control signal applied. Inanother embodiment, the filler is a gain medium amplifying the incomingradiation. The optical device can be designed employing any givenpattern of holes or slits. The focal point of the optical device canalso be adjusted by varying the wavelength of incoming light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic of a nanoslit optical device.

FIG. 1B is a schematic of an optical device with concentric rings-slits.

FIG. 1C is an SEM image of a five concentric rings sample milled into a380-nm thick gold film.

FIG. 1D illustrates experimentally-captured CCD image of the sample atthe surface z=0 μm.

FIG. 1E illustrates experimentally-captured CCD image of the sample atthe surface z=10 μm.

FIG. 2A describes the optical device operation.

FIG. 2B illustrates the numerically evaluated relationship between phaseand radius of the nanoslit when 531-nm plane waves (both linearly andcircularly polarized) are transmitted through a variable-radius hole ina 380-nm thick gold film in far field.

FIG. 3 shows a pseudo-color map of the E-field intensity of the samplecalculated from the analytical model. The color scale is normalized tothe maximal intensity. The inset shows the beam cross section at thefocal point (z=10 μm).

FIG. 4 is an SEM image of the sample before the poly(methylmethacrylate) (PMMA) spin coating process.

FIG. 5A illustrates pseudo-color cross section map of E-field intensity,obtained experimentally above the sample for x-polarized light at 488nm.

FIG. 5B illustrates pseudo-color cross section map of E-field intensity,obtained experimentally above the sample for y-polarized light at 488nm.

FIG. 5C illustrates pseudo-color cross section map of E-field intensity,obtained experimentally above the sample for x-polarized light at 531nm.

FIG. 5D illustrates pseudo-color cross section map of E-field intensity,obtained experimentally above the sample for y-polarized light at 531nm.

FIG. 5E illustrates pseudo-color cross section map of E-field intensity,obtained experimentally above the sample for x-polarized light at 647nm.

FIG. 5F illustrates pseudo-color cross section map of E-field intensity,obtained experimentally above the sample for y-polarized light at 647nm.

FIG. 6A illustrates pseudo-color cross section map based onexperimentally measured transmission through the sample ofleft-circularly-polarized light (6A) at 531 nm.

FIG. 6B illustrates pseudo-color cross section map based onexperimentally measured transmission through the sample ofright-circularly-polarized light (6A) at 531 nm.

FIG. 7 is a schematic of how the control unit 11 sends a signal 101 tothe optical device 1, which is filled and coated with a filler (PMMA orother non-linear Kerr media) 12.

FIG. 8 displays the optional adhesive layer 13 which can be coupled toone side of the optical device 1 for attachment to the core of anoptical fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A novel optical device with nanoslits or holes is disclosed. The holesare designed in the form of straight slits (defined as holes forming arectangular or linear shape) or non-uniform circular slits (defined asholes forming shapes such as ellipses, ovals, elongated circles, or anyother non-perfect round shape). In one embodiment of the presentinvention, a two-dimensional traditional nanoslit arrangement (See FIG.1A) is further developed to create a circular format. Concentricnanoslits with different widths are arranged in a pattern similar toFIG. 1B and realized, according to one embodiment of the presentinvention, by milling into a thin metal film. In this embodiment, theoptical device will cause arbitrary waveform formation to focus linearlypolarized light into a smaller circular spot. For example, whenx-polarized light is transmitted through the device, some areas of thecircular slit perform as slit waveguides upon TM-excitation (where theE-field is parallel to the radial direction), while other areas of thesame slit perform as slit waveguides upon TE-polarization (as shown inFIG. 1B). However, since the dependence of mode index on slit-width inplasmonic mode with TM-excitation is opposite to that in photonic modewith TE-polarization, such design (i.e. FIG. 1B) will not focus lightinto a circular spot unless (1) the slit widths are non-uniform or (2)the slits are non-circular (e.g. ellipses, non-perfect rounds, elongatedcircles, etc.). With such a design, the axial symmetry of the opticaldevice for arbitrarily-polarized light is lost, and the performance ofthe device becomes polarization dependent.

In order to solve the above polarization dependency and focus anarbitrarily-polarized light, while also avoiding design complexity, eachconcentric slit can instead be further separated into smaller nanoslits,or holes, in any variety of patterns, or arrays. By changing the radiusor size of the holes, one modifies the effective refractive index withinthat hole. With a pattern of holes of varying radii, the radiationtransmitted through each hole will experience a different modification(conversion), or phase change. These different phase changes of incomingradiation, which are due to varying effective refraction indices insideeach hole, combine to form a desired phase front of radiation emittedthrough all the holes in one optical device combined.

For example, and in one embodiment of the present invention, a patternof circular slits can be further discretized into a circular array ofradially equidistant, identical elliptical holes. Thus, each concentriccircular slit now consists of smaller elliptical slits arranged in acircle, equidistant from and identical to each other (i.e. a set ofconcentric arrays, or rings, of holes). The phase front of transmittedlight is controlled by adjusting the radii of the holes within eacharray. Such adjustment of hole radius to achieve phasecontrol andarbitrary waveform formation has not been explored in this field andthus is a novel aspect of the claimed invention. The design of thisspecific embodiment supports axial symmetry both globally (i.e. withregard to the ring or array of holes) and locally (i.e. with regard toeach individual hole). This allows the device to focus normallyincident, linearly-polarized light upon any polarization angle. Itshould be noted, however, that the pattern of elliptical concentricholes is only one design embodiment of the present invention, and thatany pattern of holes flowing from the formulas disclosed herein issimilarly disclosed. Global and local symmetries are possible featuresof the design, but they are not required features.

FIG. 2A schematically shows the device 1 operation. Incoming light beam2 with an arbitrary wavefront 3 is experiencing phase and intensitychange passing through the device 1. The pattern of the device 1 createsa given wavefront 5 of the outcoming light beam 4. FIG. 2A shows aparticular case when the device forms a spherical waveform focusing in afocal point 6. Note that the focal length depends on the wavelength ofthe incoming light. Therefore the distance between the device 1 and thefocal point 6 depends on the wavelength and can be changed by changingthe wavelength of incoming light.

The properties of the device 1 can be changed by a control unit 11 via acontrol signal 101, which will be described below.

In one embodiment of the present invention, each hole is filled withPoly(methyl methacrylate), or PMMA—a polymer, which, when applied, alsoforms a 200 nm-thick film on top of the gold film. The placement of gainmaterial or other nonlinear material 12, such as PMMA, inside the holescreates additional benefits. PMMA, or other gain media, increases thetransmissibility of electromagnetic radiation through each hole bycompensating losses. Any coating also creates a protective layer for theoptical device material. The insertion of other nonlinear materials(e.g. non-linear Kerr medium comprised of barium titanate) allows forthe creation of an optical device for arbitrary wave formation that isadjustable. This means, for example, that the focal length of a focusingoptical device can be changed, or tuned, by varying the wavelength ofincoming electromagnetic radiation. A similar tuning effect can beachieved for a de-focusing or hologram-forming optical device filledwith a nonlinear material. The speed of this change is similar to theKerr effect, on the order of a picosecond (in time domain, on the orderof one Terahertz, THz). The change, or tuning effect, is achieved via acontrol unit 11 connected to the optical device 1. The control unit 11sends a signal 101 to the optical device 1 which in turn changes theoperating function of the device. The signal from the control unit canbe sent very quickly and can cause a modified operating function withina matter of picoseconds. This embodiment is especially useful foradaptive optics and deformed image correction applications.

The presently claimed design of concentric arrays of holes is built onvery different operational principles than photon sieves, which havebeen used to improve the focusing property of binary Fresnel zoneplates. While photon sieves deal with multi-mode intensities diffractedby holes with wavelength-scale diameters, the design according to theclaimed invention, built on wavefront-engineering, uses single-modephase shifts obtained with holes of subwavelength-scale diameters.

Each hole milled into a metallic film can be considered a finite-lengthwaveguide with metal cladding. It should also be noted that in theoptical range, electromagnetic fields penetrate the metal claddingbecause the magnitude of the metal's permittivity is in the range of one(1) to two (2). Thus, propagating hybrid modes are excited; however,below the surface plasmon frequency, the only propagating mode in asubwavelength hole is HE₁₁ mode, and thus the presently claimed designachieves a single-mode regime (i.e. monochromatic operation).

Similar to the dependence of output phase on width for metal slits, thephase of light (i.e. electromagnetic radiation) coming through a hole inthe presently claimed invention is a function of the hole radius. Usinga three-dimensional spatial harmonic analysis method, the output phaseof a 531-nm linearly polarized light propagating through a 380-nm longhole (i.e. a hole milled into 380-nm thick gold film) from the glasssubstrate side can be simulated. A refractive index of 1.5 is used (tomatch the refractive index of Poly(methyl methacrylate) (PMMA)). Therelative phases of the output are evaluated in far field. FIG. 2 showsthe results and the relationship between phase and hole radius. As thehole radius or diameter increases and the opening becomes less confined,the mode index, and therefore the phase, increase. Such dependence issimilar to photonic mode propagation through a metallic slit. And so, byplacing holes with the largest radii towards the center and graduallydecreasing the radius of the holes in each successive concentric ring,the output light from the device can be concentrated into a tight focalspot. Additionally, it has been numerically examined and shown that thesame design achieves phase control for circularly polarized light (alsoshown in FIG. 2).

The opposite effect (i.e. diffraction) can be achieved by reversing thepattern described directly above. In this case, holes with the smallestradii are located toward the center with a gradual increase in holeradius of each successive concentric ring outward. This design willsimilarly achieve phase control for circularly polarized light inaddition to linearly polarized light. This pattern of concentric ringsformed by individual holes is only one embodiment of the presentlyclaimed invention. It should be noted that this specific designencompasses only one embodiment of the present invention, and the arrayof holes need not form any single particular pattern so long as eachindividual hole or nanoslit is either a straight slit or a non-uniformcircular slit.

After phase-radius dependence is obtained, a focusing holey opticaldevice is designed. For best results, the wavelength of the incomingradiation and the selectivity bandwidth should differ by a maximum often percent (10%), and preferably less. In order to design the device,an analytical model based on a 3D Green's function is used, in the sameway as a 2D Green's function has been used to design nanoslit lenses, toestimate the focusing performance of the device. Using the reciprocityprinciple, a dipole point source is located, emitting light with a freespace wavelength boat the desired focal point of the system, F(0, 0,f),where f is the focal length. In 3D space, the far field emitted from apoint source is proportional to a Green's function, given by:G(r)=e ^(ikr) r ⁻¹,where r=√(x ² +y ²+(z−f)²) and k=2pl ₀ ⁻¹.The phase ϕ(x, y, 0), relative to the origin O(0, 0, 0) on the x-yplane, is retrieved by taking the argument of the Green's function. Tomodel the performance of the device (i.e. the transmission profile),dipolar point sources are placed at discrete locations where the phasesare retrieved and the initial phases are assigned to the retrieved phasedelays. FIG. 3 shows the plot for intensity of an optical device forf=10 μm and l₀=531 nm. The locations of the dipolar point sourcescorrespond to the center positions of the holes shown in FIG. 4. Thefield intensity calculated by this computationally low-cost analyticalmodel captures the major features of the experimentally measuredresults, as discussed below.

The optical device can be designed as follows. A 380-nm thick gold filmis deposited on a glass substrate. Holes are milled, or perforated,through the gold film by a focused ion beam (FIB) system to create apattern similar to that portrayed by the SEM image in FIG. 4. It shouldbe noted that FIG. 4 displays only one embodiment of the presentinvention, and thus does not limit the design of placing holes in otherpatterns (e.g. variance in distance from the origin, hole radius, numberof holes, etc.). The detailed structure of the system of the presentembodiment is summarized in Table 1 below.

TABLE 1 Design parameters of the sample in FIG. 4. Concentric ringnumber 1 2 3 4 5 6 7 Distance from the 0 .8 1.2 1.6 2.0 3.3 3.63 origin(μm) Hole radius (nm) 83 76 69 62 56 83 56 Number of holes 1 13 19 26 3252 57 (per ring)After the FIB fabrication step, PMMA is spin-coated on the sample andthe sample is baked. The PMMA fills the holes and also creates a 200-nmthick uniform film on the gold surface. Such filling of the holes withpolymer decreases the cutoff radii.

Once the gold film is properly coated, transmissions through the sampleare recorded using a microscope setup with a CCD. The transmissions inthis particular experiment were performed at wavelengths of 488 nm, 531nm, and 647 nm, in two orthogonal linear polarizations. At 531 nm, thetransmissions were also recorded upon circularly polarized incidence.The resolution of the microscope stage in z axis is ±250 nm and thedepth of field of the microscope setup is about 500 nm.

In another embodiment of the present invention, the slit-ring sampleshown in FIG. 1C has concentric radii identical to the sample shown inFIG. 4, and each ring width in FIG. 1C is equal to the correspondinghole diameter of the sample shown in Table 1. Definition of opticalmeasurements and fabrication of the slit-ring sample shown in FIG. 1C isperformed using the same process as described above.

FIG. 5 summarizes the transmission measurements through the sample shownin FIG. 4. The sample is illuminated from the substrate side by apolarized laser source emitting the following types of light:x-polarized light with a wavelength of 488 nm (5A), x-polarized lightwith a wavelength of 531 nm (5C), x-polarized light with a wavelength of647 nm (5E), and y-polarized light with the same three wavelengths (488nm (5B), 531 nm (5D), 647 nm (5F)). Identical focal lengths are obtainedregardless of the polarization of the light source (x-polarized vs.y-polarized). For 488 nm illumination, the focal length is 8 μm; for 531nm illumination, the focal length is 10 μm; and for 647 nm illumination,the focal length is12 μm. This experiment shows that the optical devicefocuses incident light in accordance with the original and intendeddesign, and that such focusing is not polarization-dependent. The slightdiscrepancies between the experimental results and simulations can becaused by imperfection of the hole shape or potential damaging of thehole edge by gallium during FIB fabrication. Nevertheless, theanalytical model can be utilized for rapid prototyping, with potentialuses in further optimization of devices through full-wave finite elementmodeling. The insets corresponding to each image of FIG. 5 show apseudo-color x-y map of the E-field intensity at the focal point. Asshown, the focus profiles do not depend on the incident polarization.The focus spots are also circular, as distinguished from the focus spotsresulting from a slit-ring design, as shown in FIG. 1E. Finally, andsignificantly, the results show that by changing the incidentwavelength, the focal distance of the same optical device can beshifted.

FIG. 6 shows the measured results for circularly polarized incidentlight with a wavelength of 531 nm. FIG. 6A displays the result of aleft-circularly polarized incident light; FIG. 6B displays the result ofa right-circularly polarized incident light. As expected from theinformation provided by FIG. 2, the focal lengths and focusing profilesfor left- and right-circular polarizations are very similar to theresults shown in FIGS. 5C and 5D (i.e. the results of x- and y-polarizedincident light at 531 nm). These results further show the incidentpolarization independence of the optical device claimed herein.

The above disclosure serves to demonstrate the performance of thepresently claimed polarization-independent holey optical device. First,by changing the radius or size of subwavelength holes milled into ametallic film, which act as single-mode waveguide elements, the phase oflight transmitted through the holes in the film can be controlled.Second, by milling specific patterns of subwavelength holes withdifferent radii to form a desired phase front, specifically desiredtransmission properties are achieved. The patterns of subwavelengthholes can be redesigned not only to control focusing and focal distancesbut also to create de-focusing devices and hologram-forming devices(based on the same principles). Third, the holey optical devicestructure can be filled with a gain medium such as PMMA to compensatelosses. It should be noted, however, that other nonlinear materials canbe used to fill the holes instead of PMMA. While filling the holes witha gain medium compensates losses, a nonlinear material filling makes thefocal length of the device tunable by the incident power. Finally, itshould also be noted that the presently claimed invention need not bemilled into a gold film. Any material with a negative permittivity canbe used. Examples of other materials include, but are not limited to,aluminum, silver, copper, silicon carbide, and titanium nitride (andother ceramics).

The design according to the present disclosure is simple and planar,thus allowing for high throughput fabrication methods, such asnanoimprint lithography or laser dynamic forming. If a ceramic is used(e.g. titanium nitride), even higher powered lasers can be used due tothe higher melting point. The compact design of the device is helpful inachieving focal distances on the order of one micrometer for on-chipdevices as well as fiber-coupled devices. Additional applications of thepresently disclosed device include electronic circuits and sensors, aswell as fiber optics. With respect to those devices filled with gainmedia or other nonlinear material for adjustable tuning based onincoming radiation wavelength, applications include adaptive optics fordistorted imaging correction.

Gain media filler applied to compensating losses and amplification oflight inside the holes can be made for example of a common organic dye,such as R800, R6G, and R101, and their COTS analogs embedded in apolymer host, such as for example, PVA.

The description of a preferred embodiment of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

The invention claimed is:
 1. A method of making an optical devicecomprising: forming a plurality of holes with varying radii milledvertically into a film, wherein said holes form a pattern, wherein aradius of each hole determines an effective refractive index for saidhole, said effective refractive index modifying a phase and an intensityof an incoming electromagnetic radiation as the radiation propagatesthrough said hole, wherein the device is configured to be operatingequally for each linearly polarized radiation simultaneously, whereinthe each linearly polarized radiation is normally incident on thedevice.
 2. The method of claim 1, wherein a focal distance of theoptical device, which is operating as a lens, is controlled byadjustment of a wavelength of the incoming electromagnetic radiation. 3.The method of claim 2, wherein a focal distance of the optical device isin the range from one to twenty micrometer.
 4. The method of claim 1,further comprising a filler applied to the device after the holes aremilled, the filler filling the holes.
 5. The method of claim 4, whereinthe filler is a non-linear Kerr medium.
 6. The device of claim 5,wherein the Kerr medium changes an intensity of incident electromagneticradiation and controls the device operation in real time.
 7. The methodof claim 6, further comprising a control unit, the control unit applyinga signal to the device thus changing an operating function of the devicein real time with THz frequency.
 8. The method of claim 4, wherein thefiller is a gain medium serving for amplification of the radiationintensity and compensation for plasmonic losses.
 9. The method of claim1, wherein said film is a pure metal film comprised of gold, aluminum,silver, or copper.
 10. The method of claim 1, wherein said film is aceramic film made of silicon carbide.
 11. The method of claim 1, whereinsaid film is a non-stoichiometric ceramic film made of titanium nitride,or zirconium nitride.
 12. The method of claim 1, wherein the phasechange increases as the hole radius becomes larger.
 13. The method ofclaim 1, wherein said film comprises gold, silver, or copper.
 14. Amethod of making an optical device comprising: milling a pattern of aplurality of holes into a film, wherein said pattern comprises holes ofvarying width corresponding to a desired effective refractive indexwithin each hole, wherein said effective refractive index of eachindividual hole modifies a phase and an intensity magnitude of theradiation propagating through said individual hole, and outputting anoutput radiation, wherein said output radiation propagates through eachindividual hole and experiences various refractive modifications,wherein an entirety of the film comprises a pure metal, wherein a devicecontaining the film is configured to be operating equally for eachlinearly polarized radiation simultaneously, wherein the each linearlypolarized radiation is normally incident on the device.
 15. The methodof claim 14, wherein the radiation is a radiation in an optical range.16. The method of claim 14, wherein prior to milling, depositing thefilm on a fiber core covered with an adhesive layer.
 17. The method ofclaim 14, further comprising: applying a filler to the holes after theholes are milled, the filler filling the holes, the filler is anon-linear Kerr medium.
 18. The method of claim 17, wherein the Kerrmedium changes an intensity of incident electromagnetic radiation andcontrols the device operation in real time.
 19. The method of claim 14,wherein the pure metal film comprises gold, aluminum, silver, or copper.20. A method of making an optical device comprising: an electromagneticradiation source; forming a plurality of holes with varying radii milledvertically into a film, wherein said holes form a pattern, wherein aradius of each hole is configured to receive an incoming electromagneticradiation through the electromagnetic radiation source, wherein thedevice is configured to be operating equally for each linearly polarizedradiation simultaneously, wherein the each linearly polarized radiationis normally incident on the device.